Overview of Transcriptional Control of Gene Expression

• Focus on transcriptional control in prokaryotes, mainly through operons.
• Key topics:
  • General structure and components of operons (e.g., lac operon).
  • Differentiating between lac operon and tryptophan operon.
  • Predicting outcomes with mutants of protein-coding genes and regulatory sequences.

Operons and Gene Organization

• Operons: Groups of genes transcribed together as a single mRNA strand.
  • Operons allow bacteria to efficiently regulate genes involved in metabolic pathways.
• Coupled Transcription and Translation: In prokaryotes, transcription and translation happen simultaneously in the cytoplasm.

Lac Operon Components

• Structure: Comprises three structural genes:
  • lacZ: Codes for beta-galactosidase (breaks down lactose).
  • lacY: Codes for permease (facilitates lactose transport).
  • lacA: Codes for transacetylase (role in metabolism).
• Regulatory Regions:
  • Promoter: Site where RNA polymerase binds to initiate transcription.
  • Operator: Site regulating transcription by binding the repressor protein.
  • Regulatory Gene (lacI): Codes for the repressor protein that inhibits transcription in the absence of lactose.

Functioning of the Lac Operon

• Presence of Lactose: Lactose acts as an inducer by binding to the repressor, causing it to release from the operator, allowing transcription to proceed.
• Use of XGal: An analog of lactose used in experiments to test beta-galactosidase activity, yielding a blue color if active.
• Mutations: Mutants in the lac operon can help illustrate functions of genes and regulatory elements:
  • i− Mutant: Repressor cannot bind, leading to continuous expression of enzymes regardless of lactose presence.

Negative Regulation of Lac Operon

• Default state: Gene expression is on (transcription active), turned off only by a specific regulator (the repressor).
• Mutant Types: Various mutants affect operon functionality:
  • Z−: Non-functional beta-galactosidase.
  • Y−: Non-functional permease.
  • A−: Non-functional transacetylase.
• Wild Type vs. Mutants:
  • Without lactose: Repressor binds operator, transcription is blocked.
  • With lactose in wild type: Transcription proceeds, resulting in enzyme production.

Glucose and Lac Operon Regulation

• Glucose Preference: E. Coli prefers glucose over lactose; high glucose levels inhibit lac operon expression irrespective of lactose availability.
• cAMP as a Regulatory Molecule:
  • Low glucose → high cAMP → cAMP binds CAP (catabolic activator protein) → enhances transcription.
  • High glucose → low or no cAMP → CAP unable to assist RNA polymerase effectively, reducing transcription levels.

Comparison with Tryptophan Operon

• Tryptophan Operon: Involved in the biosynthesis of tryptophan (negative feedback regulation).
  • Absence of tryptophan: genes are active to produce more tryptophan.
  • Presence of tryptophan: binds repressor, blocks transcription at the operator.

Eukaryotic Gene Regulation

• More complex than prokaryotic systems; involves multiple levels:
  • Transcriptional regulation, post-translational modifications, RNA processing.
• Chromatin Modification: Essential for gene expression regulation:
  • Histone acetylation, methylation, and phosphorylation modify histone tails to control chromatin structure.
• Covalent Modifications:
  • Acetylation neutralizes positive charge, relaxing DNA-histone interaction for active transcription.
  • Other modifications can either activate or silence genes depending on their location and extent.
• Chromatin Remodeling: SWISNF complex plays a critical role in making DNA accessible for transcription.

Summary

• Operons are key to understanding prokaryotic gene expression control, particularly the lac and tryptophan operons.
• Understanding mutations helps clarify the regulatory mechanisms.
• Eukaryotic gene regulation is multifaceted, emphasizing chromatin structure and modifications.